Sulfuric acid Ammonia NucleaTIon And GrOwth model (SANTIAGO)

Nucleation from sulfuric acid and ammonia is an important source of new particles and cloud condensation nuclei (CCN) on a global scale (Dunne et al., 2016). This is due to an acid-base stabilization mechanism where the ammonia molecules lead to reduced evaporation rates for the mixed system of sulfuric acid and ammonia (Kirkby et al., 2011; Kürten et al., 2016). The slower evaporation rates enhance the chance that a formed cluster will grow to a stable aerosol particle that can ultimately become a cloud condensation nuclei. In order to model the formation of new particles and CCN, including the very first steps of cluster formation, evaporation rates for the most important clusters as a function of temperature are required.

The process model SANTIAGO (Sulfuric acid Ammonia NucleaTIon And GrOwth model) uses evaporation rates derived from more than one hundred CLOUD nucleation experiments covering a wide range of conditions (temperature between 208 and 292 K, ammonia mixing ratios from ~1 pptv to ~1 ppbv and atmospherically relevant sulfuric acid concentrations; Kürten, 2019). The model allows calculating particle formation rates and concentrations for a wide size range (from the monomer of sulfuric acid up to a particle diameter of several hundred nanometers). It further includes the effect of coagulation (self-coagulation and coagulation with pre-existing particles, i.e., the condensation/coagulation sink) and yields the growth rate of particles as function of the particle size. The input parameters include the sulfuric acid concentration, ammonia mixing ratio, temperature, and the relative humidity.